Researchers investigate whether warming has changed the northern distribution of a bird. At northern sites they conduct one-hour surveys using the same methods. The number of surveys increases each year, but sites are always selected randomly from the same area.
| Survey year | Mean winter temperature / $^{∘\circ∘}C\mathrm{C}C$ | Surveys in each season | Total birds in winter | Total birds in summer |
|---|---|---|---|---|
| 1 | 2 | 5 | 20 | 200 |
| 2 | 3 | 10 | 80 | 390 |
| 3 | 4 | 20 | 320 | 820 |
| 4 | 5 | 40 | 1120 | 1600 |
Which conclusion distinguishes the seasonal evidence from what the study cannot establish?
Summer use of northern sites has increased, but the results do not establish whether winter temperature caused this change.
Winter use of northern sites has increased, but the results do not establish whether winter temperature caused this change.
Winter use of northern sites has increased, and the stable summer rate establishes that winter temperature caused this change.
Summer use of northern sites has increased, and the rising winter counts establish that winter temperature caused this change.
40 exam-style questions on AQA GCSE Biology 7.2 Organisation of an ecosystem, covering 7.2.1 Levels of organisation, 7.2.2 How materials are cycled, 7.2.3 Decomposition, and 7.2.4 Impact of environmental change (HT only). Each one has a worked solution and a mark scheme showing where the marks go.